The General Aim of this project is to determine possible consequences of maternal exposure to organochlorine pesticides, such as dieldrin, on development of serotonin (5-HT) neurons and their receptors in offspring. The rationale for examining the effects of dieldrin on development of the serotonergic system is previous evidence that dieldrin strongly inhibits the growth and survival of embryonic 5-HT neurons in vitro. Dieldrin appears to cause these effects by acting as as a potent GABAA receptor antagonist to block the trophic effects of GABA. The Specific Aims are designed to determine whether maternal administration of different doses of dieldrin or the classical GABAA receptor antagonist bicuculline from embryonic day E12-E17 alters development of 5-HT neurons and their receptors in prenatal and postnatal rat brain. In Specific Aim 1, effects of treatments on development of 5-HT neurons in offspring will be determined at embryonic day E18 and postnatal days (PND) 0, 10, 30 and 60. Standard (ABC peroxidase) immunocytochemistry with specific 5-HT antibodies will be used to assess effects on numbers of 5-HT immunoreactive cell bodies and the general distribution of their axonal projections. Semi-quantitative radiolabel immunocytochemistry ([125I]protein A immunobinding) will be used to measure relative changes in 5-HT immunoreactivity in anatomically matched brain sections. In Specific Aim 2, effects of treatments on expression of multiple 5-HT receptor subtypes will be determined using assays to measure changes in receptor mRNA and protein at E18 and PND 0, 10, 30 and 60. Quantitative competitive RT-PCR with primers and internal standards specific for 5-HT1A, 5-HT2A, 5-HT2B, 5-HT2C and 5-HT3 receptor sequences will be used to measure changes in receptor mRNA in dissected brain regions. [125I]protein A immunobinding with specific antibodies to 5-HT1A, 5-HT2A, 5-HT2B, 5-HT2C and 5-HT3 receptors will analyze relative changes in receptor immunoreactivity in specific regions of anatomically matched brain sections. These studies will provide insights into the risk potential of maternal exposure to organochlorine pesticides for neural development in their offspring. This is an important question because of the suspected involvement of dysfunctional ontogeny of the serotonergic system in the etiology of developmental disabilities and mood disorders.